wifi: mt76: mt7915: Fix VHT beamforming capabilities with DBDC

Do not hardcode beamformee 4 space-time streams support, because
DBDC can only support up to 2.

Fixes: 00b2e16e ("mt76: mt7915: add TxBF capabilities")
Signed-off-by: NNicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: NFelix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/init.c

@@ -735,7 +735,8 @@ void mt7915_set_stream_vht_txbf_caps(struct mt7915_phy *phy)
 
 	*cap |= IEEE80211_VHT_CAP_SU_BEAMFORMEE_CAPABLE |
 		IEEE80211_VHT_CAP_MU_BEAMFORMEE_CAPABLE |
-		(3 << IEEE80211_VHT_CAP_BEAMFORMEE_STS_SHIFT);
+		FIELD_PREP(IEEE80211_VHT_CAP_BEAMFORMEE_STS_MASK,
+			   sts - 1);
 
 	*cap &= ~(IEEE80211_VHT_CAP_SOUNDING_DIMENSIONS_MASK |
 		  IEEE80211_VHT_CAP_SU_BEAMFORMER_CAPABLE |